Maruyama et al., in their Japanese Patent Application Publication No. HEI 11-38410 (A) laid open for public inspection on Feb. 12, 1999, disclose use of a semi-transmissive liquid crystal display device in order to reduce liquid crystal display device power dissipation or consumption. The liquid crystal display device of Maruyama et al. is operated as a transmissive liquid crystal display device by the use of a cold-cathode fluorescent lamp (CCFL) as a backlight source, when the liquid crystal display device is operated in a dark environment. In a light environment, it does not use the cold cathode fluorescent lamp, but uses a white reflective plate to reflect environmental light so that the liquid crystal display device can be operated as a reflective liquid crystal display device.
In order to reduce power dissipation, Kurumizawa discloses in his Japanese Patent Application Publication No. HEI 11-101980 (A) laid open for public inspection on Apr. 13, 1999, a liquid crystal display device using a cold cathode fluorescent lamp and chemiluminescence. The liquid crystal display device of Kurumizawa uses a cold cathode fluorescent lamp as a backlight source when an electronic apparatus which employs the liquid crystal display device is operated from an AC power supply, while it uses a bag containing a chemiluminescent mixture solution as a backlight source when the electronic apparatus is operated from a DC battery.
The semi-transmissive liquid crystal display device disclosed in Japanese Patent Application Publication No. HEI 11-38410 (A) can use a DC power supply battery for a longer time when it is operated as a reflective liquid crystal display device in a light place. The semi-transmissive liquid crystal display device uses a cold cathode fluorescent lamp when it is used in a dark environment and, therefore, requires higher brightness. However, its display is less bright than and, therefore, inferior in quality to an ordinary transmissive liquid crystal display device when it is operated from the same power supply level as the ordinary transmissive liquid crystal display device, because light transmission is restricted due to its semi-transmissive nature. Accordingly, the liquid crystal display device of Maruyama et al. requires higher power to provide the same brightness as the ordinary transmissive liquid crystal display device.
The liquid crystal display device employing a cold cathode fluorescent lamp and chemiluminescence disclosed in Japanese Patent Application Publication No. HEI 11-101980 (A) requires a bag containing chemically luminescent mixture solution to be inserted into the liquid crystal display device. This liquid crystal display device is not economical because, once the bag starts emitting light, the light emission cannot be interrupted. In addition, a user of the liquid crystal display device must take a chemiluminescent bag or bags with him or her, and must take a trouble of disposing the used bag.
The inventors have recognized that power dissipation of a liquid crystal display device and an electronic apparatus with the liquid crystal display device can be reduced by selectively using an efficient light source providing relatively high brightness (hereinafter referred to as high-brightness light source) and an efficient light source providing relatively low brightness (hereinafter referred to low-brightness light source) as a backlight source for the liquid crystal display device depending on brightness required for the liquid crystal display device. The inventors have also recognized that it is desirable to use a combination of high-brightness and low-brightness light sources with a light guide plate having prisms or grooves formed between the adjacent prisms arranged as to realize efficient use of light emitted from whichever light source selected and to realize uniform brightness distribution.
An object of the present invention is to provide a light guide plate having a structure suitable for use with a combination of two light sources providing high brightness and low brightness.